1. Field of the Invention
This invention relates generally to light emitting devices, and more particularly to a light emitting diode (LED) or laser diode coated with phosphors.
2. Description of the Related Art
Solid state light sources have been available for many years. The colors have been predominantly red, orange, and green. Within the last several years blue LEDs have also been produced. The combination of red, green and blue LEDs has enabled the generation of white light with a complicated 4 lead package. However, the different LEDs have different lumen vs life curves causing the color to change with life. In the Fall of 1996, Nichia of Japan introduced a new white LED, Product No. NSPW310AS. This product uses yttrium aluminum garnet doped with cerium to convert blue emission from the LED into a very broad band yellow emission. The emission peak is at 580 nm with a full width at half maximum of 160 nm. The phosphor is coated directly on the LED. The entire device is encapsulated in a clear plastic lens. The emission contains enough orange emission to produce white light at a color temperature of about 8000.degree. K, a color rendering index (CRI) of about 77, and a device luminous efficacy of approximately 5 lumens per watt (lm/w) of input electric power.
It is well know that tastes in lighting characteristics can vary by geographical region. For example, in some regions of the world, high color temperatures are preferred, while in other regions lower color temperatures are desired. Similarly, in some regions, CRIs in the mid-nineties are preferred, while in other regions CRIs in the mid-eighties are preferred. Hence, it is desirable to adapt the output characteristics of a lamp to suit particular preferences.
The color temperature of the white LED produced by Nichia can be lowered by using more phosphor, however, the system efficiency is also decreased. There is a need for a phosphor-coated LED having a higher device luminous efficacy; there is also a need for such an LED having a lower color temperature.